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Sensitive Determination of PAH-DNA Adducts

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Sensitive Determination of PAH-DNA Adducts View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Matrix Design for Matrix-Assisted Laser Desorption Ionization: Sensitive Determination of PAH-DNA Adducts M. George, J. M. Y. Wellemans, R. L. Cerny, and M. L. Gross* Midwest Center for Mass Spectrometry, Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska USA K. Li and E. L. Cavalieri Eppley institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, 600 South 42nd Street, Omaha, Nebraska USA Two matrices, 4-phenyl-cw-cyanocinnamic acid (ICC) and 4-benzyloxy-cu-cyanocinnamic acid (BCC), were identified for the determination of polycyclic aromatic hydrocarbon @‘AH) adducts of DNA bases by matrix-assisted laser desorption ionization. These matrices were designed based on the concept that the matrix and the analyte should have structural similarities. PCC is a good matrix for the desorption of not only PA&modified DNA bases, but also PAHs themselves and their rnetabolites. Detections can be made at the femtomolar level. (1 Am Sot Mass Spectrum 1994, 5, 1021-1025) he success of matrix-assisted laser desorption quire sensitive detection methods at the low picomole ionization (MALDI) for the sensitive detection of and mid-femtomole level, respectively [6]. T an analyte depends on the nature of the matrix. One method for detection that also gives structural Rational design of a matrix requires understanding of information is fast-atom bombardment (FAB) coupled the mechanism of MALDI. The mechanism is not fully with tandem mass spectrometry [6-81. Unfortunately, understood. Nevertheless, empirical criteria for matrix the combination lacks sensitivity because there is in selection have been suggested 111. Two important ones, efficiency of FAB desorption and interference from in our view, are (1) high molar absorptivity and (21 matrix-derived ions. Improvements can be made by miscibility with the analyte in the solid phase. derivatizing the modified base 191 and by using coaxial The question to be answered is how does one assure continuous-flow FAB [lo]. miscibility in the solid state? We propose that miscibil- MALDI ionization may offer high sensitivity for ity is best achieved when the analyte and matrix modified DNA bases as it has for peptides Ill] and molecules have similar structures. In this paper, we proteins [12J. Encouragement comes from the success- expand upon the guidelines proposed earlier [l] and ful application of MALDI coupled with time-of-flight describe the design and evaluation of a matrix molecule mass spectrometry to other molecules such as poly- for the specific problem of desorbing polycyclic aro- mers [13-151 and oligosaccharides [16]. For DNA and matic hydrocarbon (PAHI adducts of DNA bases. Our RNA, success has been realized with both large [17] argument at this stage is qualitative, and we do not and small [I81 oligonucleotides, nucleotides [19], and address quantitatively the solubility of an analyte in one modified nucleoside, which was detected at the the matrix. femtomolar level [20]. MALDI coupled with Fourier Some PAH materials are carcinogenic [Zl because transform mass spectrometry also has been demon- they are metabolically activated to form electrophiles strated for the determination of modified nucleosides that attack nucleophilic sites on DNA bases. Three produced by attack of PAH diol epoxides on DNA, but mechanisms of activation of PAHs have been pro- the required quantities of material thus far are in the posed: (1) formation of dioI epoxide [3], (2) formation 20-pm01 range [Zl, 221. of a radical cation [4], and (3) formation of benzylic esters [5]. Investigations of the relative importance of these mechanisms in in vitro and in vivo studies re- Experimental The synthesis of the PAH-modified DNA bases was Address reprint requests to Professor Michael L. Gross, Editor-in- published elsewhere [23]. Siphenyl carboxylic acid, Chief, Journal of Mass Spectromctry, Department of Chemishy, Washington University. 1 Brooking Drive, St. Louis, MO 63130. anthracene-9-carboxylic acid, and 4-hydroxy-o-cyan* 8 1994 American Society for Mass Spectrometry Received June 9,1994 1044~0305/94/$7.00 Revised July 13,1994 Accepted August 8,1994 1022 GEORGE ET AL. J Am Sac Mass Spectram 1994,5,1021-1025 cinnamic acid (4HCC) were purchased from Aldrich 5.26 (s, lH), 6 7.27 (d, 2H), S 7.38-7.43 (m, 3H), 6 Chemical Co. (St. Louis, MO). 4-Benzyloxy-o-cyanocin- 7.50-7.52 Cd, 2H), S 8.09-8.12 Cd, 2H), 6 8.24 ts, lH), namic acid (BCC) (mp 204205 ‘C) was synthesized by for BCC] with a GE Omega spectrometer (General reacting benzyl bromide (1.21 g) with 4HCC (0.9 g) in Electric, Fremont, CA) operating at 300 MHz. Nega- methanol. Potassium hydroxide (0.8 g) was added to tive-ion FAB that used 3nitrobenzyl alcohol as matrix deprotonate the 4HCC. 4-Phenyl-cu-cyanocinnamic acid produced abundant [M - HI- ions of m / z 278.0821 (KC) was synthesized following the literature proce (calculated for C,,H,,NO,, 278.0817) and 248.0710 dure [24], mp 243-244 “C (lit. [24] mp 243-245 “C; see (calculated for C,,H,,NO,, 248.0712) for BCC and structures). KC, respectively. The compounds were pure at least as could be determined by thin-layer chromatography and melting point. The spectra were obtained with a COOH Kratos MS-50 three-sector mass spectrometer (Kratos Analytical, Ramsey, NJ) equipped with an argon FAB @$Q o-o-H gun. The MALDI-TOF experiments were carried out on a Anthracene-9.carboxylic acid ~ipheny~carboxyli~ acid BenchTOF Bruker instrument (Bruker-Franzen Ana- lytik GMBH, Bremen, Germany). A nitrogen laser beam (337 nm, 20-kW peak laser power, spot size of 1 mm*, and 1.25-ns pulse width) was used to desorb the sam- HO CH=C c @@H-Cc -o- ples. The ions were accelerated with a potential of 25 kV. 4HCC PCC Samples of benzo[ a]pyrene-6-N7-guanine (BP&N7- Gua), 1 [25], dibenzd a,f]pyrene-lo-N7-guanine (DBP- N7-Gua), 2 1261, dibenzo[ a,l]pyrene-IO-N7-adenine [26],and dibenzo[ a,l]pyrene-8,9-diol (DBP-8,9-diol), 3 [27] were obtained from synthesis as previously de- scribed. Dibenzo[a,I]pyrene-11,12-diol (DBP-11,lZ ECC diol), 4, was purchased from CHBMSYN Science Labo- ratories (Lenexa, KS), benzotelpyrene (B[e]P), 5, was purchased from Sigma Chemical Co. (St. Louis, MO), and dibenzoJa,l]pyrene (DBP), 6, was obtained from the National Cancer Institute Chemical Carcinogen Repository (Bethesda, MD) (see structures). Stock solu- tions (11 pmol/pL for BP-6-N7Gua, 2.5 pmol/FL for DBP-N’-Gua, 100 nmol/pL for dibenzo[ a,Z]pyrene-lO- N7-adenine, 10 pmol/pL for DBP-8,9-dial, 10 pmol/pL for DBP-ll,IZ-diol, I nmol/l_~L for B[e]P, and 10 pmol/gL for DBP) were made by dissolving the analyte in methanol. Saturated solutions of the matrices were made in a mixture (40:60) of acetonitrile and water (1% trifluoroacetic acid). Samples were pre- pared by mixing 1 PL of matrix and variable volumes (up to 1 FL) of analyte on the probe tip and allowing them to crystallize prior to irradiation with the laser. It was not possible to use calibration plots to deter- mine the limits of detection because the signals pro- duced by MALDI were highly variable. As an expedi- ent, the lowest amounts of analytes required for repro- ducible observation of their molecular ions (signal-to- noise ratio of at least 3:l) for three sample loadings were taken as detection limits. These limits were estab- lished by loading progressively lower and lower amounts of analyte. Fifty to sixty laser shots were signal-averaged for each spectrum. A blank of each a 4 matrix was run to identify the matrix peaks that might PCC and BCC were characterized by ‘H NMR JO confound an analyte peak. Data acquisition was con- 7.40-7.55 (m, 3H), S 7.75-7.80 (d, 2H), 6 7.87-7.95 (d, trolled by using the Bruker SUN data system and 2H), S 8.17-8.22 (d, 2H), S 8.37 (s, 1H) for KC and S XTOF software. J Am Sot Mass Spectrom 1994, 5,1021-1025 MATRIX DESIGN FOR MALDI 1023 Results and Discussion (Met-i)’ The structures of the PAH-modified DNA bases (for I 402 example benzo[ alpyrene-6-N7-guanine (BP&N7-Gua), 1, and dibenzo[ a,I]pyrene-lo-N’-guanine (DBP-N7- Gus), 2 have nonpolar aromatic ring systems on one end and a polar nitrogen heterocycle on the other. In an attempt to match the nature of the matrix with that of the analyte, we selected a set of aromatic com- pounds-biphenyl carboxylic acid, anthracene-9- carboxylic acid, 4-benzyloxy-ct-cyanocinnamic acid (BCC), and 4-phenyl-a-cyanocinnamic acid (PCC)--as Figure 1. MALDI-TOF mass spectrum of 11 pmol of BP-6-N7- candidate matrices for this class of PAH-modified Gus obtained by using the 4HCC matrix. The [M + HI+ ion is of m/z 402. The asterisks (‘1 indicate matrix ions. bases. These matrix candidates are similar in structure to the adducts because they have a polar group on one end of the molecule and aromatic rings on the other. The matrix, 4-hydroxy-a-cyanocinnamic acid (4HCC1, to give a high abundance of radical cations when one of the common MALDI matrices used for desorb- biphenyl carboxylic acid was the matrix. A modified ing peptides, is included in this study for comparison.
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